Heretofore, there has been known a stator fabricated by preparing separate cores each comprising an arcuate yoke and a pole extending radially from the yoke, winding conductive wires around the poles of the separate cores with insulating bobbins interposed therebetween, to produce coils on the poles, and assembling the separate cores with the coils thereon into an annular assembly.
For the purpose of increasing the performance of rotary electric machines which incorporate such a stator, research and development efforts have been made to increase the number of wires of the coils wound around the separate cores to achieve an increased space factor in the slots between the poles. In one of such efforts, it has been proposed to increase the space factor by winding thin wires in spaces between the coil wires, as disclosed in Japanese Laid-Open Patent Publication No. 09-140079.
According to Japanese Laid-Open Patent Publication No. 09-140079, as shown in FIG. 17 of the accompanying drawings, a coil comprises a thick wire 902, a thin wire 904, a thick wire 906, and a thin wire 908 which are wound in multiple layers around a bobbin 900 and have ends connected to three terminals, not shown. The thick wire 902 and the thin wire 904 are wound in a pair, and the thick wire 906 and the thin wire 908 are wound in a pair. The thin wires 904, 908 have turns disposed in substantially square spaces defined between four turns of the thick wires 902, 906. The thick wires 902, 906 and the thin wires 904, 908 have a diameter ratio of 1:0.4
According to the coil described in Japanese Laid-Open Patent Publication No. 09-140079, the turns in the respective layers of the thick wires 902, 906 as main wires are held in alignment with each other. Therefore, the turns in one layer of the thick wires 902, 906 are kept in point contact with the respective turns in a lower layer of the thick wires 902, 906. The stacked turns of the thick wires 902, 906 are thus relatively unstable positionally. The turns in the second and subsequent layers of the thick wires 902, 906 are particularly unstable because they are more likely to slip into the spaces between the turns in the lower layers of the thick wires 902, 906 as they are wound under higher tension.
The coil has a considerably high space factor with the thin wires 904, 908 being disposed in the gaps between the thick wires 902, 906. However, there is a demand in the art for a stator having a higher space factor.